Imported image as plane does not have accurate colors
I believe you are on the right track... 

Switch from Filmic to standard but not on the image or material like you did. 

I actually didn't realize you could do it image by image like that but I think you need to switch it on the actual world scene. 

Look under the "Scene" tab where you switch from EEVEE to Cycles... at the bottom look for "Color Management" and "View Transform" then switch that from Filmic to Standard.